What happened after the epidemic? Equine influenza surveillance sheds light on sources and seasonal risk in the United Kingdom.

Abstract

BACKGROUND: The epidemiology of equine influenza (EI) in the United Kingdom has not been systematically described since the 2019 epidemic. OBJECTIVES: To summarise UK EI surveillance (2020-2024), quantify outbreak seasonality and assess movement-related sources. STUDY DESIGN: Retrospective observational analysis of national surveillance and horse importation data. METHODS: Epidemiological data for laboratory-confirmed EI cases in the United Kingdom were collated. Outbreaks (EI-infected premises) were defined as one or more laboratory-confirmed cases on the same premises within a 4-week period. Monthly outbreak counts were analysed using negative binomial regression with year, calendar-quarter and ordered quartiles of 1-month lagged Irish exports to the United Kingdom by equid commodity code. A subset of Q4-2022 sales-related EI outbreaks were mapped. RESULTS: Epidemiological data were available for 149 cases on 126 premises. Outbreaks displayed a repeatable late-year pattern: Q4 (October to December) accounted for 52% (65/126), with a 3.25-fold higher per-month rate than the rest of the year. Over 75% (95/126) of premises reported a new arrival within &#x2264;2&#x2009;weeks; 56% (28/50) of index new-arrival cases with recorded origin came from Ireland. Q4 incidence exceeded Q1 (incidence rate ratio [IRR] 6.9, p&#x2009;<&#x2009;0.001) and years 2021-2024 incidence exceeded 2020 (IRRs 4.5-5.6, p&#x2009;<&#x2009;0.001). Adding lagged Irish imports other than pure-bred breeding animals, improved fit, attenuated the Q4 effect (IRR&#x2009;=&#x2009;3.9, p&#x2009;<&#x2009;0.001) and identified higher import quartiles as predictors (quartile-3: IRR 4.5, p&#x2009;<&#x2009;0.001; quartile-4: IRR 3.7, p&#x2009;<&#x2009;0.001). MAIN LIMITATIONS: Under-ascertainment, UK-wide exposure data versus Great Britain-only outcomes, COVID-19 suppression of movements/testing in 2020. CONCLUSIONS: EI in the United Kingdom in 2020-2024 was characterised by a notable October to December risk window and strong links to horse movements. Trade in non-pure-bred horses aligns with outbreak timing and partly explains the seasonal excess. Control measures should prioritise vaccination of new arrivals, post-arrival quarantine and strengthened biosecurity during transport.